1. Field of the Invention
The present invention relates to electrophotographic image forming apparatuses in which image forming is carried out by developing, using toner, a latent image that has been formed on a photosensitive member, transferring a toner image to a transfer material (sheet), and performing fixing.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a configuration of a conventional color image forming apparatus.
This color image forming apparatus is provided with two cassette feeding units 1 and 2, and one manual paper feeding unit 3, and sheets S are selectively fed from each of the feeding units 1, 2, and 3. The sheets S are loaded on cassettes 4 and 5 or a tray 6 of the feeding units 1, 2, and 3, and are drawn out in order from the topmost sheet due to rotation of a pickup roller 7. Then, only the topmost sheet of the sheets S that have been drawn out by the pickup roller 7 is separated by a pair of separation rollers, which is constituted by a feed roller 8A and a retardation roller 8B, and sent to registration rollers 12, the rotation of which is being paused. In this case, the sheet S that has been fed from the paper supply cassette 4 or 5, which has long distances to the registration rollers 12, is sent to the registration rollers 12 by being relayed through multiple pairs of conveying rollers 9, 10, and 11. When the leading edge of the sheet S that has been sent to the registration rollers 12 in this manner hits a nip of the registration rollers 12 and forms a predetermined loop shape, movement of that sheet S is temporarily paused. A diagonal traveling condition of the sheet S is corrected by the formation of this loop.
Downstream from the registration rollers 12, a long intermediate transfer belt (endless belt) 13, which is an intermediate transfer member, is arranged in a tensioned state on a drive roller 13a, a secondary transfer opposing roller 13b, and a tension roller 13c, and from a cross-sectional perspective is set in a substantially triangular shape. The intermediate transfer belt 13 rotates in a clockwise direction in the diagram. A plurality of photosensitive drums 14, 15, 16, and 17, on which color toner images of different colors are formed and carried, are arranged in order along the rotational direction of the intermediate transfer belt 13 on an upper surface of the horizontal section of the intermediate transfer belt 13.
It should be noted that in the rotation direction of the intermediate transfer belt 13, the most upstream photosensitive drum 14 carries a magenta color toner image, the next photosensitive drum 15 carries a cyan color toner image, the photosensitive drum 16 carries a yellow color toner image, and the photosensitive drum 17 carries a black color toner image. First, exposure of a laser light LM commences on the most upstream photosensitive drum 14 based on image data of a magenta component to form an electrostatic latent image on the photosensitive drum 14. This electrostatic latent image is made visible by the magenta color toner supplied from a developer 23. Next, exposure of a laser light LC commences on the photosensitive drum 15 based on image data of a cyan component to form an electrostatic latent image on the photosensitive drum 15. This electrostatic latent image is made visible by the cyan color toner supplied from a developer 24. Numeral 22 denotes a scanner unit, which is an exposure means of the photosensitive drums 14 to 17.
Next, after a predetermined time has elapsed from the commencement of exposure of the laser light LC to the photosensitive drum 15, exposure of a laser light LY commences on the photosensitive drum 16 based on image data of a yellow component to form an electrostatic latent image on the photosensitive drum 16. This electrostatic latent image is made visible by the yellow color toner supplied from a developer 25. Next, after a predetermined time has elapsed from the commencement of exposure of the laser light LY to the photosensitive drum 16, exposure of a laser light LB commences on the photosensitive drum 17 based on image data of a black component to form an electrostatic latent image on the photosensitive drum 17. This electrostatic latent image is made visible by the black color toner supplied from a developer 26. It should be noted that primary chargers 27, 28, 29, and 30 for uniformly charging the photosensitive drums 14 to 17 are provided at a circumference of the photosensitive drums 14 to 17. Further still, cleaners 31, 32, 33, and 34 are arranged for removing toner that has adhered on the photosensitive drums 14 to 17 after transfer of the toner images.
In the process of rotating clockwise, the intermediate transfer belt 13 passes successively through transfer portions between the photosensitive drums 14, 15, 16, and 17 and their corresponding transfer chargers 90, 91, 92, and 93. Due to this, the toner images of each of the colors magenta, cyan, yellow, and black are transferred onto the intermediate transfer belt 13 superimposed on each other.
Meanwhile, the registration rollers 12 commence rotating with a timing that matches the positions of the toner image on the intermediate transfer belt 13 and the leading edge of the sheet. Due to this, the sheet S, which has been sent to the registration rollers 12 and had its diagonal traveling condition corrected, is sent to a secondary transfer portion T2, which is a contact portion between a secondary transfer roller 40 on the intermediate transfer belt 13 and the secondary transfer opposing roller 13b, and the toner image is transferred onto the sheet S.
In this manner, the sheet S, which has passed through the secondary transfer portion T2, is sent to a fixing unit 35. Then, due to a process of passing through a nip portion formed by a fixing roller 35A and a pressure roller 35B in the fixing unit 35, the sheet S is subjected to heat by the fixing roller 35A and pressure by the pressure roller 35B, and the transferred toner image is fixed onto the sheet.
The sheet S that has passed through the fixing unit 35 and undergone the fixing process is sent by a pair of conveyance rollers 36 to a pair of discharge rollers 37, and moreover is discharged outside the apparatus onto a discharge tray 38.
This image forming apparatus is capable of a double side mode of image forming. Hereinafter, the configuration of the image forming apparatus is further described in accordance with a flow of the sheet S during double side mode.
When double side mode is specified, the sheet S that has passed through the fixing unit 35 and undergone the fixing process is set to an inversion path 59 via a vertical path 58. In this case, as flapper 60 opens the vertical path 58 and the sheet S is conveyed by pairs of conveyance rollers 36, 61, and 62, and a pair of reverse rollers 63.
The pair of reverse rollers 63 rotate in reverse at a time point at which the trailing edge of the sheet S, which is conveyed in the direction of arrow a by the pair of reverse rollers 63, has passed a point P, and the sheet S is conveyed in the direction of arrow b with its trailing edge side now in front. Due to this operation, the surface of the sheet S where the toner image has been transferred becomes the upper side. It should be noted that a flapper 64 is provided for the point P that makes it possible for the sheet S to advance from the vertical path 58 to the inversion path 59, but makes it impossible for the sheet S to enter from the inversion path 59 to the vertical path 58. Further still, a detection lever 65 is provided for detecting that the trailing edge of the sheet has passed the point P.
The sheet S that has been conveyed in the direction of arrow b due to the reverse rotation of the pair of reverse rollers 63 is sent into a re-feeding path 67. Then it is relayed by multiple pairs of conveyance rollers 68 inside the re-feeding path and the pair of conveyance rollers 11 and sent to the pair of registration rollers 12 to undergo image forming again. In this manner, the sheet S is sent to the secondary transfer portion T2 after its diagonal traveling condition has been corrected by the registration rollers 12. Then a second instance of image forming is carried out based on image data stored in an image memory (not shown) on which main scanning direction and sub-scanning direction scaling ratio correction has been carried out. Thereafter, the sheet S undergoes the same processing as for one-side image forming, and is discharged outside the apparatus.
Next, description is given of the scanner unit 22 in which the photosensitive drums are exposed.
FIG. 2 is a diagram that schematically shows a configuration of one color portion of the scanner 22 shown in the convention example of FIG. 1.
The electrophotographic image forming apparatus is provided with an exposure unit that irradiates laser light onto a photosensitive drum 215 (corresponding to each of the photosensitive drums 14, 15, 16, and 17) as shown in FIG. 2 so as to form a latent image on the photosensitive drum 215 corresponding to the inputted image data. The exposure unit is provided with a laser light source 210 for emitting diffused laser light. The laser light emitted from the laser light source 210 is converted to a parallel laser light L1 via a collimator lens 211. The laser light L1 is irradiated onto a polygon mirror 213 that is being rotationally driven by a scanner motor 212. Then, the laser light L1 that has been irradiated onto the polygon mirror 213 is reflected by the polygon mirror 213 and guided to an f-θ lens 214. The laser light that has passed through the f-θ lens 214 is made to perform combined scanning on the photosensitive drum 215 at a uniform velocity in the main scanning direction, and a latent image 216 is formed on the photosensitive drum 215 due to the scanning of the laser light, that is, due to the scanning operation. The commencement of the scanning operation of the laser light is detected by a beam detect sensor (hereinafter referred to as BD sensor) 217. The laser light source 210 is forcibly turned on at a time aligned with the commencement of scanning of the laser light on the photosensitive drum 215. In this way, in the period in which the laser light source 210 is forcibly turned on, the BD sensor 217 detects the laser light that has been inputted by being reflected by the polygon mirror 213, and outputs a beam detect signal (hereinafter referred to as a BD signal), which is a reference signal for the timing of writing in image forming for each main scanning line.
However, in the above-described conventional example, the scaling ratio in the sub-scanning direction is fixed, and therefore there have been the following problems. For example, the following two large problems involve driving the intermediate transfer belt.
Due to problems such as geometrical shape deviation between the drive roller 13a and the idler roller 13c, the velocity of the intermediate transfer belt 13 changes from time to time. For this reason, positional differences are produced in successively formed images on the intermediate transfer belt 13 compared with the ideal image forming position in the movement direction of the intermediate transfer belt 13, that is, in the sub-scanning direction on the sheet. In particular, in the case of an apparatus capable of forming a full color image by superimposing four color images as in the conventional example, there is a problem that poor color registration occurs and image quality deteriorates. Some of the main causes of this are as follows.
(1) A movement velocity V of the intermediate transfer belt prescribed by a drive roller driven at a constant angular velocity ω and having a radius r, with a thickness h of the intermediate transfer belt is expressed as follows.V=(r+h)×ω  expression (1)
When an eccentricity Δr is superposed to the drive roller 13a, a fluctuation ΔV of the movement velocity V of the intermediate transfer belt 13 prescribed by the drive roller 13a is expressed as follows.ΔVω=Δrω×ω  expression (2)Here ω indicates angular velocity (rotation period of the drive roller).
Due to the velocity fluctuation ΔVω in the rotation period of the drive roller 13a, a positional displacement in each image of the colors is produced at the rotation period of the drive roller 13a. 
(2) Furthermore, change in the movement velocity of the intermediate transfer belt prescribed by the drive roller is produced also by fluctuation in the thickness direction extending over the entire circumference of the intermediate transfer belt. As a result, the images of each color on the sheet, which have been transferred as a batch from the intermediate transfer belt, are displaced from their ideal positions and image quality deteriorates. There is also a problem of fluctuation in the positions of images formed on a plurality of sheets.
Now assume that when the drive roller of a radius r is driven at a constant angular velocity ω, thickness fluctuation Δh is present extending along the entire circumference of the intermediate transfer belt that winds around the drive roller and has a thickness h. In this case, a fluctuation ΔVL in the movement velocity V of the intermediate transfer belt driven by the drive roller is expressed by expression (3).ΔVL=ΔhL×ω  expression (3)
Here L indicates the entire circumferential length of the intermediate transfer belt.
FIGS. 3 and 4 schematically express an ideal case of linear velocity fluctuation of the belt prescribed by the drive roller and the positional displacement relationship of images that are thereby formed, and a case including the aforementioned problems. In FIGS. 3 and 4, the exposure timing of each exposure device is shown. The movement velocity of the intermediate transfer belt is indicated by a time t on the horizontal axis and a linear movement velocity v of the belt is shown on the vertical axis. The scanning lines of each color formed on the intermediate transfer belt are shown in parallel in the main scanning direction, with these being shown as they are written in a time series.
FIG. 3 shows an ideal case in which the intermediate transfer belt moves at a constant velocity V. Here, a case is shown in which a movement time gap is set corresponding to installation spacings between the image forming apparatuses of each of the colors YMCK on the intermediate transfer belt, which moves at constant velocity, and each portion of writing in the main scanning direction is set in regular spacing times in the sub-scanning direction. As a result, it is evident that each of the YMCK color scanning lines is written having regular spacings in the sub-scanning direction without displacement.
In contrast to this, FIG. 4 shows a case in which the velocity of the intermediate transfer belt changes due to the thickness of the intermediate transfer belt and eccentricity of the drive roller. Small AC component fluctuation of the velocity fluctuation of the intermediate transfer belt shown by the solid line corresponds to an eccentric cycle of the drive roller, and the large undulating component shown by the dashed line pertains to velocity fluctuation corresponding to a cycle of thickness unevenness of the intermediate transfer belt.
In this case, even when the scanning lines of each color are formed having regular spacings in the sub-scanning direction, there is misalignment in the sub-scanning spacings of the scanning lines corresponding to the amount of velocity fluctuation in the intermediate transfer belt. Furthermore, as a result of this condition occurring for each color respectively, poor color registration is produced among the YMCK colors.
Japanese Patent Laid-Open No. 2004-102039 proposes a method to counter this problem in which the rotation velocity of the polygonal mirror scanner motor is controlled to carry out sub-scanning direction scaling ratio correction. However, there are limitations in the speed of response of the rate of rotation of the motor. For this reason, although this method is effective against long period scaling ratio unevenness, it is has a poor effect against positional displacement in short period scaling ratio unevenness.
Furthermore, there is also a problem of front to back displacement in images during double sided printing. Ordinary sheets such as paper are known to expand or contract slightly (2% or less) due to changes in the amount of water contained in the sheet due to the application of heat during fixing. In other words, image expansion or contraction occurs along with expansion or contraction of the sheet after forming and fixing an image on the front side of the sheet. After this, when image forming is carried out on the reverse side of the sheet while the expansion/contraction has not returned to normal, an image is formed and fixed on the expanded or contracted sheet. After a certain time after this, when the amount of water in the sheet is restored and the image scaling ratio of the front side image has returned to its original size, the image on the reverse side conversely contracts or expands undesirably, which produces slight inconsistencies in the scaling ratios between the front and reverse sides.
Along with higher image quality in recent years, a need has arisen to correct these slight inconsistencies in the scaling ratios. As mentioned above, methods have also been proposed to counter this problem by controlling the rotation velocity of the polygonal mirror scanner motor to carry out sub-scanning direction scaling ratio correction. However, this necessitates changing the rotational speed of the motor between sheets such that an unnecessary time for changing speed between sheets must be maintained. This has an adverse effect on printing efficiency.